The manufacture of semiconductor devices is a complex process that often requires over 200 process steps. Each step requires an optimal set of conditions to produce a high yield of semiconductor devices. Many of these process steps require the use of fluids to, inter alia, etch, expose, coat, and polish the surfaces of the devices during manufacturing. In high purity fluid applications, the fluids must be substantially free of particulate and metal contaminants in order to prevent defects in the finished devices. In chemical-mechanical polishing slurry applications, the fluids must be free from large particles capable of scratching the surfaces of the devices. Moreover, during manufacturing there must be a stable and sufficient supply of the fluids to the process tools carrying out the various steps in order to avoid process fluctuations and manufacturing downtime.
Since their introduction to the semiconductor market in the 1990s, bulk fluid distribution systems having vacuum-pressure engines have played an important role in semiconductor manufacturing processes. Because these systems are substantially constructed of inert wetted materials, such as perfluoroalkoxy (PFA) and polytetrafluoroethylene (PTFE), and because they use an inert pressurized gas as the motive force for supplying the fluids, they do not substantially contribute to particulate and metal contamination of the process fluids. In addition, a single bulk fluid distribution system can provide a continuous supply of process fluid at a sufficient pressure to multiple process tools. Thus, the advent of vacuum-pressure fluid distribution systems served an important need in the semiconductor market.
For many reasons, bulk fluid distribution systems (e.g. o-ring failures, valve failures, or contaminated incoming fluid) include filters in the fluid supply line. However, an abrupt change in the flow rate of the fluid through the filters causes hydraulic shock to the filters which results in a release of previously filtered particles into the fluid thereby causing a spike in the particle concentration. Although maintaining a minimum flow rate of the fluid through the filters helps reduce particulate release, the problem is not eliminated. Accordingly, pressure and flow fluctuations of the fluid can result in fluctuations of the particle concentration in the fluid, which may lead to defects in the semiconductor wafers.
Moreover, as discussed above, fluid distribution systems often supply many tools. When a tool demands process fluid, the fluid is pumped from the supply line which causes the pressure of the fluid in the supply line to drop by about 5 to about 25 psi. As will be discussed further below, typical fluid distribution systems having vacuum-pressure engines cause pressure fluctuations in the supply line which may adversely affect the flow and purity conditions of the fluid supplied to the tools. Accordingly, there is a need for a fluid distribution system that minimizes or eliminates pressure and flow fluctuations of the fluid in the supply line.
FIG. 1a depicts a standard vacuum-pressure fluid distribution system used to supply process fluids to semiconductor process tools. Other types of vacuum-pressure fluid distribution systems are described in U.S. Pat. Nos. 5,330,072 and 6,019,250, which are incorporated herein by reference.
With reference to FIG. 1a, a vacuum-pressure fluid distribution system typically includes two pressure-vacuum vessels 101 and 103. Each vessel is equipped with at least two fluid level sensors 105, 107, 109 and 111 (e.g. capacitive sensors). Sensors 105 and 109 monitor a low fluid level condition in vessels 101 and 103, respectively; and sensors 107 and 111 monitor a high-fluid level condition in vessels 101 and 103, respectively. The process fluid from fluid source 113 enters vessel 101 through two-way valve 115 and enters vessel 103 through two-way valve 117. The fluid exits vessel 101 through two-way valve 119 and exits vessel 103 through two-way valve 121. Upon exiting vessel 101 or vessel 103, the fluid flows through the bulk process fluid supply line 123.
During a fill cycle, a vacuum-generating device 125 (e.g. an aspirator or venturi) creates a vacuum in vessel 101 to draw in the fluid. When the fluid flows into vessel 101 during a fill cycle, two-way valves 115 and 127 are open and three-way valve 129 is in position “A”. When the vacuum is operated on vessel 101, any gas in vessel 101 flows to an exhaust (not shown) as the fluid from the fluid source 113 is drawn into the vessel. When the fluid reaches level sensor 107 (e.g. a capacitive sensor), valves 115, 127 and 129 deactivate and the vacuum stops.
During a dispense cycle, an inert gas 131, such as nitrogen, flows through “slave” regulator 133 and through position “B” of three-way valve 129 into vessel 101. Vessel 101 is initially pressurized to a predetermined value and then valve 119 opens allowing the fluid to flow under the force of the inert gas pressure through valve 119, through the filters (not shown) and into the bulk fluid supply line 123. The vessel 101 dispenses the fluid until it reaches low level sensor 105 at which point valve 119 closes and the fill cycle begins again.
During operation, vessels 101 and 103 alternate between fill and dispense cycles such that when vessel 101 is filling, vessel 103 is dispensing. During a fill cycle in vessel 103, valves 117 and 127 are open and valve 137 is in position “A”. During a dispense cycle in vessel 103, inert gas 131 flows through slave regulator 135 and port “B” of valve 137 to pressurize the fluid in vessel 103 and drive it through valve 121 to supply line 123. At the end of a dispense cycle in vessel 103, the vessels switchover so that vessel 103 begins a fill cycle and vessel 101 begins a dispense cycle. Notably, the vacuum-generating device 125 is configured so that the vessels fill faster than they dispense to provide a continuous flow of fluid to the supply line 123.
In the system shown in FIG. 1a, a manually-adjustable master regulator 141 is facilitated with a gas, such as compressed dry air, from a high pressure gas source 139. The master regulator 137 sends a constant gas pilot signal to both slave regulators 133 and 135 which thus provide a constant inert gas pressure to valves 129 and 137, respectively. The pressure supplied to each valve 129 and 127 is the same. Accordingly, during a dispense cycle of either vessel 101 or 103, the inert gas pressure supplied to each vessel is constant and the same.
A problem with the system of FIG. 1a is that it does not maintain a stable pressure of the fluid in the supply line 123. FIG. 1b shows a simplified illustration of how the pressure of the fluid in supply line 123 fluctuates over time. Losses due to process tool demands, fittings, piping and other parts present in a complex fluid distribution system were not accounted for in this illustration. During operation of system 100, as a vessel dispenses from its high sensor to its low sensor, the pressure in the supply line 123 decreases by an amount equivalent to the loss of the head pressure of the fluid between the high and low sensors. The head pressure is defined as the pressure resulting from the weight of the fluid in the vessel acting on the fluid in the supply line. When the vessels switchover the vessel beginning its dispense cycle starts full with fluid up to its high sensor, and the same pressure that was applied to the vessel that just completed its dispense cycle, is applied to the dispensing vessel. Thus, when the vessels switchover the pressure of the fluid in the supply line spikes or increases by an amount equivalent to the head pressure of the newly dispensing vessel.
There have been efforts to improve the system of FIG. 1a by actively controlling the pressure of the fluid in the supply line. FIG. 2a shows a modified vacuum-pressure system 200. System 200 is substantially similar to system 100 except that an electro-pneumatic master regulator 241 is used instead of manually-adjustable regulator 141. As in system 100, the electro-pneumatic master regulator 241 of system 200 is facilitated with a gas, such as compressed dry air, from a high pressure gas source 239. The system of FIG. 2a also includes a sensor 245 to monitor the pressure at a mid-point in the supply line 223. Like the system of FIG. 1a, vessels 201 and 203 alternate between vacuum fill and pressure dispense cycles, and master regulator 241 provides the same pneumatic signal to both slave regulators 233 and 235.
During a dispense cycle, the inert gas pressure applied to the fluid in the dispensing vessel 201 or 203 is adjusted based upon a signal from the pressure indicator 245. Considering a simplified fluid distribution system with no process tool demands or other pressure losses, the inert gas pressure supplied to the dispensing vessel 201 or 203 while it is dispensing increases to compensate for the loss in head pressure between the high and low sensors (207, 211 and 205, 209, respectively) of the vessel.
Although system 200 prevents a pressure decrease due to head loss in the dispensing vessel, it does not provide stable pressure control of the fluid in the supply line 223. FIG. 2b is an illustration of how the pressure in supply line 223 can fluctuate over time in a distribution system free from process tool demands or other pressure losses. During operation, when the vessels switchover the master regulator 241 continues to send the same signal (or pressure requirement) to the vessel beginning its dispense cycle as it was sending to the vessel that just completed its dispense cycle. Accordingly, when the vessels switchover there is a spike in the pressure in the supply line 223 equivalent to the change in head pressure between the high and low sensors of the vessel that just completed its dispense cycle. As a result, the system 200 actively attempts to decrease the pressure of the fluid in the supply line 223 and continues to adjust the pressure until it reaches a predetermined setpoint. Thus, a problem with the system 200 is that the pressure of the fluid in the supply line 223 oscillates until it reaches a steady state as shown in FIG. 2b. 
In addition, another problem with system 200 is that it continually adjusts the pneumatic signal to the slave regulator of the non-dispensing or standby vessel. Thus, the slave regulator for the non-dispensing vessel incurs significant wear and tear on the slave regulator of the standby vessel.
Accordingly, there remains a need in the semiconductor industry for improvements to fluid distribution systems including providing stable control of the flow conditions of the process fluid without causing wear and tear on the component parts.